As a method for measuring a current value of ion beams without interrupting the beams with high accuracy, several studies have been reported conventionally (see non-patent document 1). This method measures a beam current value by detecting a magnetic field which a beam current generates using a sensor which is referred to as SQUID which uses a Josephson coupling method which is an extremely sensitive magnetic field sensor. The SQUID includes one (RF-SQUID) or two (DC-SQUID) Josephson junctions in a super-conductive ring, and measures a magnetic flux which penetrates the super-conductive ring using a magnetic flux quantum (2.07×10−15 Wb) as a scale.
In the above-mentioned document, the SQUID which uses a low-temperature superconductive body which is operated at a temperature of liquefied helium is used. Further, the beam current measuring device has a main part thereof constituted of a detecting part which detects a magnetic field corresponding to a beam current, a magnetic flux transmitting part which transmits a magnetic flux to a measuring part, the measuring part which includes a superconductive element which responses to the transmitted magnetic flux and a feedback coil which allows a feedback current such that the feedback current cancels a change of the magnetic flux which penetrates the superconductive element, and a magnetic shielding part made of a superconductive body and having a gap which magnetically shields the detecting part, the magnetic flux measuring part and the measuring part from an outer space which includes a space in which ion beams flow.
The detecting part is a coil which is formed by winding a super conductive line on a core made of a soft magnetic core and induces a superconductive current into the coil by collecting magnetic fields which are generated by the beam current by the soft magnetic core. Then, this superconductive current induced in the coil is transmitted to the coil which is arranged close to the SQUID. That is, in response to the change of the beam current, the superconductive current which flows in the coil is changed thus changing a quantity of magnetic flux which flows in the SQUID. The feedback coil is provided for allowing the feedback current to flow so as to cancel the change of the magnetic flux. The feedback current is proportional to the change of the beam current value and the change quantity of the beam current value can be determined by measuring the feedback current.
Recently, a measuring method of the beam current value using a high-temperature superconductive body has been studied (see non-patent document 2). According to the method described in this non-patent document 2, a cylinder which has a surface thereof coated with a high-temperature superconductive body constitutes a detecting part. However, on an outer peripheral surface of the cylinder, a bridge portion which has a portion thereof made of a high-temperature superconductive body is formed. A beam current which penetrates the center of the cylinder induces a surface shielding current on a surface of the cylinder. Here, the surface shielding current concentrates on the bridge portion. Then, a magnetic flux which is generated by the concentrated surface shielding current is measured by a SQUID. The SQUID which is used in this method uses the high-temperature superconductive body and is operable at a liquefied nitrogen temperature or more.
The beam current measuring device which uses the former SQUID made of the low-temperature superconductive body can measure the beam current with a noise band corresponding to several nA.
On the other hand, the beam current measuring device which uses the latter SQUID made of the high-temperature superconductive body has an advantage that the measuring device can be operated with only liquefied nitrogen or a freezer, a noise band is considered to be large, that is, around several μA (see non-patent literature 2). Further, a drift on a zero point is considered to be large and there has been a drawback that, in an actual measurement for several tens seconds or more, the measuring device can only measure the beam current substantially corresponding to 10 μA or more. To the contrary, there has been a report that by designing the magnetic shielding such that the sensitivity of the high-temperature superconductive SQUID is optimized, ion beams of 1.8 μA are successfully measured (see patent document 1, patent document 2, non-patent document 3). Here, the noise band corresponding to 0.5 μA. In this manner, recently, the studies and developments of the high-temperature superconductive SQUID have been in progress.
In other non-destructive measuring method, a DC current transformer is used. The noise band is approximately 0.5 μA to several μA although the noise band depends on the design of the magnetic shielding.
Non patent literature 1: Superconducting Quantum Interference Devices and Their Applications (Walter de Gruyter, 1977) p. 311,IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-21, NO. 2, MARCH 1985,Proc, 5th European Particle Accelerator Conf., Sitges, 1996 (Institute of Physics, 1997) p. 1627, Publication of Japan society of physics Vol. 54, No. 1, 1999Non patent literature 2: IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, MARCH 2001 p. 635Non patent literature 3: CNS annual reportPatent literature 1: Japanese Patent Application 2003-155407Patent literature 2: Japanese Patent Application 2003-331848